A variety of techniques have been developed for collecting and interpreting data concerning the electrical activity of the heart. Some techniques use external medical devices (EMDs) in the clinical setting, and others use implantable medical devices (IMDs).
Implantable cardiac monitors, such as the MEDTRONIC® Reveal™ insertable loop recorder, have also been developed and clinically implanted that employ the capability of recording cardiac electrogram (EGM) data for subsequent interrogation and uplink telemetry transmission to an external programmer for analysis by a medical care provider. The recorded data may be retrieved using an external programmer operated by a medical care provider. The programmer may include the ability to display the retrieved EGM data and/or perform processing and analysis functions on the retrieved EGM data. Stored segments of data can be transmitted via telemetry transmission to an external device for further analysis when a telemetry session is initiated. Aspects of the Reveal™ insertable loop recorder are disclosed in commonly assigned PCT publication WO98/02209 and in U.S. Pat. No. 6,230,059, the disclosures of which are hereby incorporated by reference in their entirety.
Scatter-plots, sometimes referred to as Lorentz or Poincaré plots, have been used to plot EGM and electrocardiograph (ECG) data. For example, U.S. Pat. No. 5,622,178 issued to Gilham describes a system and method for dynamically displaying cardiac interval data using scatter-plots. Consecutive R-R intervals form a coordinate pair (e.g., the “x” and “y” coordinates) of a point that is plotted on the scatter-plot. Subsequent points are plotted by “sliding” one heartbeat to the next group of successive heartbeats (i.e., the second, third, and fourth heartbeats) and plotting the next pair of R-R intervals. FIG. 1 illustrates an example of this technique in plotting R-R interval data on a scatter plot. Such scatter-plots may provide a visual indication of the variability of the parameter plotted.
Published Patent Application No. 2002/0065473 compares a number of consecutive R-R interval differences with a predetermined value to measure ventricular rate stability as part of a method to detect atrial fibrillation. A counter is incremented every time the measured R-R interval difference exceeds a threshold value, and classifies a rhythm as AF when a threshold total number is reached. Published Patent Application No. 2004/0092836 discloses an AF detection algorithm that uses a Cluster Signature Metric (CSM) that is based on the two-dimensional distribution of “first order lag” of R-R interval differences. Each of the above references is incorporated by reference herein in its entirety.
In atrial fibrillation (AF), the atria depolarize at an elevated rate that is highly irregular. The irregular nature of the ventricular response during AF is characterized by fluctuations in the intervals between ventricular contractions. In atrial flutter (AFL), the atria beat at an elevated rate that is highly regular, and a certain portion of the atrial depolarizations may be conducted to the ventricles in a predictable pattern. There are many instances where it is desirable to be able to diagnose intermittent spontaneous cardiac arrhythmias, particularly AF and AFL, in ambulatory patients. Atrial rate may serve as a criterion for distinguishing between AFL and AF. For example, AFL (types I and II) may typically occur at rates that can range from about 220 to about 450 bpm, and AF typically occurs at rates greater than about 300 bpm. However, since a significant range of overlap exists between AF and AFL, and since the range of overlap may be even greater in patients taking anti-arrhythmic drugs and/or in elderly patients, a method of distinguishing them that does not rely solely on rate is needed.
A method and system for performing scatter-plot analysis to classify cardiac rhythms is desired which minimizes computational resource requirements and allows for real-time processing of cardiac signals. A method of using such real-time data to affect therapy selection decisions (e.g., by an IMD) is also desirable.